Traditionally, X-band EPR spectroscopy of small nitroxide radicals such as Tempo (2,2,6,6-tetramethylpiperedine-1-oxyl) or DTBN (di-tert-butyl nitroxide) has been employed to study partitioning in phospholipid membrane systems. Because of a lack of spectral resolution, these data provided no or very little information on molecular dynamics of the probe molecules in the membrane. It was shown recently that g-value resolution of W-band EPR substantially facilitates such studies (Smirnov, Smirnova and Morse II, P.D., Biophys. J. 68, 2350, 1995). However, the question remains whether dynamics behavior of the probe in the membrane still can be treated under a one compartment model, i.e., neglecting the membrane architecture. Improved sensitivity of the Illinois W-band EPR instrument and better handling of aqueous samples allowed us to measure high signal-to-noise spectra from Tempo/DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine) system. Least-squares simulation analysis of these spe ctra shows that W-band EPR is sensitive to the statistical distribution of the probe within the membrane. We also demonstrate how this distribution becomes affected by anesthetics.